In the production of plastic films for use in, for example, food wrap applications, it is known to coextrude two or more polymeric materials to produce a clear coextrusion multilayer film or extrusion coated film. The selection of the particular polymeric materials for coextrusion is dictated by the desired properties of the coextruded multilayer film or extruded coated film. Thus, it is known, for example, to coextrude multilayer films comprising layers of polystyrene and one or more layers of a natural rubber, styrene-butadiene copolymer, polybutadiene, poly-isoprene, vinyl polymers, conjugated diolefin polymers, and others.
It is also known to put various additives and/or agents into one or more of the layers in coextruded films to achieve certain properties. For example, it is known to add fillers, such as calcium carbonate, magnesium carbonate, calcium sulphate, aluminum oxide and hydrates thereof, clay, aluminum sulphate, silica, talc, mica, wollastonite, vermiculite, asbestos fibers, and glass fibers, to a layer of styrene polymer composition in a thermoplastic polymeric laminate. U.S. Pat. No. 4,101,050, issued July 18, 1978 to Buckler, et al., teaches such addition of fillers to produce laminates which can be formed and molded into plastic products such as containers having desirable combination of properties.
It is also known to add SiO.sub.2 in a monolayer film rather than in a multilayer coextruded film. However, the addition of SiO.sub.2 in monolayer films has been in methods which disperse the SiO.sub.2 throughout the full thickness of the product film. Furthermore, the inventors believe that such addition of SiO.sub.2 has been for use as a slip additive to increase slipperiness of the film or for achieving anti-blocking properties.
U.S. Pat. No. 4,595,625, issued June 17, 1986 to Crass et al., teaches a sealable transparent multilayer film comprising a base layer and at least one sealable layer. The sealable layer comprises from about 0.1 to 1.0% by weight of a silicate, such as aluminum silicate, having the shape of platelets. The sealable layer also, however, is required to contain several other materials, including an olefin resin composition, a low molecular weight resin which is compatible with the olefin resin composition, a propylene homopolymer, and a polydiorganosiloxane. These added materials tend to increase the cost and complexity of the film.
U.S. Pat. No. 4,147,291, issued Apr. 3, 1979 to Akao, et al., teaches the use of inorganic fillers, including clay, as a light-intercepting agent or as a pigment in a low density polyethylene film used as part of a multilayer construction useful as a packing bag.
U.S. Pat. No. 4,132,700, issued Jan. 2, 1979 to Stephens, deceased, et al., teaches a process for preparing polyvinyl chloride compositions containing acid-treated talc particles or kaolinite particles. The acid treatment is required for achieving the desired increased tensile strength but results in some discoloration of the plastic film produced. Ammonia or amine treatment of kaolinite particles is taught in Stephens, et al., for increasing tensile strength without discoloration of polyvinyl chloride compositions.
U.S. Pat. No. 4,666,772, issued May 19, 1987 to Schinkel, et al., is related to an opaque heat-sealable multilayer film consisting of a biaxially oriented base layer of polypropylene polymers (BOPP) containing 3 to 15% by weight of additives, such as calcium carbonate, silica, Na-Al silicate and/or titanium dioxide. The additive in Schinkel, et al., must be incompatible with the propylene polymer of the BOPP base layer of the film so as to produce the desired opaque product. Schinkel, et al., does not teach the use of kaolin clay in clear films or the advantage of improved slittability resulting therefrom, but rather is directed toward producing heat-sealable opaque films, containing at least one monoaxially-oriented layer.
U.S. Pat. No. 3,221,226, issued Nov. 30, 1965 to Kennedy, et al., teaches the production of an electrical capacitor comprising a smooth roll of polyester film containing a finely divided, dispersed inorganic pigment. The pigment of Kennedy, et al., can include kaolin (china clay). However, the purpose of the addition of pigment to the polyester film of Kennedy, et al., is to produce desirable slipperiness therein, allowing single sheets of film to be fed into packaging equipment without wrinkling.
U.S. Pat. No. 2,527,329, issued Oct. 24, 1950 to Powers, et al., teaches the topical treatment of the outer surfaces of textile fabrics with an aqueous colloidal solution of silica. The silica solution of Powers, et al., is a silica sol applied to fibers to form a film of silica on the surface of the fibers, whereby certain physical properties of the fibers are improved.
One disadvantage of many of the films described in the above references is the high tendency of the rolled films to telescope during handling, on the slitter, or when the film roll is inserted into a packaging machine. By "telescoping" herein is meant a lateral displacement of the film on the roll, in which the surfaces of adjacent layers all slide to one side or the other in an uncontrolled or undesired manner or extent, as a result of which the edges of the film are no longer positioned exactly above one another. Instead, each layer is displaced in one direction or the other by a small amount, and therefore the whole roll protrudes from one side of the mandrel. Telescoping can result when rolls are bumped against something or when they are held in a slightly inclined position. Thus a need exists for a process for improving the slittability and subsequent wind up into a roll of clear plastic film.
It is therefore an object of the present invention to produce a multilayer clear BOPP coextruded film which can be readily slit and subsequently wound onto a roll with little or no telescoping. It is also an object of this invention to provide a process for improving the slittability and subsequent winding into a roll of plastic multilayer coextrusion film.